1. Field of the Invention
This invention relates to air-laid fibrous webs, and to method and apparatus for manufacturing same.
2. Description of the Prior Art
Absorbent fibrous members having a thick center region therein have been utilized in sanitary products, such as disposable diapers, as exemplified in U.S. Pat. Nos. 2,890,700, issued to Lonberg-Holm; 3,430,629, issued to Murphy; 3,509,604, issued to Furbeck; 3,598,680, issued to Lee; and 3,682,761, issued to Lee et al. The absorbent members described in all of the above patents are internal components of a sanitary product. Such absorbent members normally are not sufficiently self-sustaining to be utilized alone, or in conjunction with only a backing layer, to form a complete sanitary product, such as a disposable diaper. Stating this another way, separate facing and backing sheets normally are required to provide structural support for such absorbent members. Therefore, the sanitary product constructions disclosed in the above patents must be fabricated by utilizing fairly complicated converting equipment in which the absorbent member is sandwiched between opposed facing and backing sheets, and in which the facing and backing sheets are adhered together. In addition, one or both of the facing and backing sheets is usually an adhesively bonded web to begin with. Therefore, adhesive is not only utilized to form a self-sustaining facing and/or backing web, but additional adhesive is required to adhere the facing and backing webs to each other. It is highly desirable to decrease the amount of adhesive necessary to fabricate sanitary products to thereby reduce their cost of manufacture. Reduced cost is an important consideration in products such as disposable diapers which are intended for disposal after a single use.
The absorbent internal members disclosed in the patents to Murphy U.S. Pat. No. 3,430,629, Furbeck U.S. Pat. No. 3,509,604, Lee U.S. Pat. No. 3,598,680, and Lee et al U.S. Pat. No. 3,682,761, all have a thick central region which extends the full length of the absorbent member, and thinner flanking side portions. This construction provides a conformable, snug fit in the thigh region of a wearer while providing a thicker, more absorbent section in the perineal region where it is needed to retain body fluids. However, the forward and rearward regions of the absorbent members include a portion of the thick central region as well as a portion of the thinner flanking side portions. Diapers having a forward and rearward region with thickness variations along the transverse extent thereof do not provide as conformable a waste band region as a diaper in which the forward and rearward regions have a substantially uniform thickness extending for the full transverse extent thereof.
The absorbent member employed in the diaper disclosed in Lonberg-Holm includes a heavier basis weight medial portion than flanking end and side portions thereof. This absorbent member is formed from a plurality of plies of crepe paper wadding which must be properly positioned relative to each other on suitable converting equipment. Lonberg-Holm discloses that the layers or plies could include a mass of fibrous material which is uniform in character and of a varying thickness such that there are no physically distinct layers. However, this discussion implies the separate formation of fibrous members which are married together to achieve sufficient interlocking between fibers to render the layers physically indistinct. Such a method of forming an absorbent member still requires suitable equipment for precisely aligning various fibrous webs together in order to achieve the varying thickness construction. In addition, as stated above, Lonberg-Holm is not directed to an absorbent member which can be utilized either by itself as a sanitary product, or in conjunction with only a backing layer. Lonberg-Holm requires an absorbent member which is structurally held in place by being disposed between separate facing and backing layers in the conventional manner employed today in disposable diapers.
Prior art apparatus and method for forming air-laid, profiled fibrous webs have included means for establishing a non-uniform basis weight distribution in the cross-machine direction of web formation. However, such apparatus do not include means for affecting a non-uniform basis weight distribution in the machine direction of web formation by initially depositing different weights of fibers in different areas. For example, the Lee et al apparatus U.S. Pat. No. 3,682,761 includes a plurality of stationary vacuum chambers disposed behind a moving foraminous surface upon which an air-lay fibrous web is formed. Therefore, any transverse section of the forming surface passes over the same bank of vacuum boxes, and accordingly, the machine direction basis weight of the fibrous web formed in any transverse region of the forming surface will be substantially the same. However, by properly adjusting the vacuum levels in the various banks of vacuum boxes, a cross-machine direction basis weight variation can be achieved.
In the Lee and Furbeck apparatus separate continuous air-laid webs are formed, and are deposited in partial overlapping relationship with each other to provide a centrally disposed region having a greater basis weight than flanking side portions. This arrangement for forming a profiled fibrous web also results in the formation of a thickened region extending for the full length of the absorbent air-laid web.